Irrigation is needed in many arid areas for supplying water to architectural vegetation. Greenery is often planted around homes, commercial buildings and apartment developments to provide an attractive outdoor appearance. In order to minimize the manual labor of such irrigation and to avoid loss of plants when irrigation is forgotten, clocks are often used to control electrically actuated valves which supply water to the irrigation system. Such clocks have been developed to a point where one or more days of the week can be selected for sprinkling. Furthermore, the time of day and duration of sprinking can also be selected. Such clocks sometimes have a plurality of terminals which can be connected to different water control valves for the control thereof. Such terminals are often referred to as stations. The clock usually sequentially energizes the stations, and the clock can be adjusted so that each station has its own individual adjustment of on time. In this way, irrigation of a number of different areas, each served by a station and a sprinkler valve, can be employed to deliver water in accordance with the program determined by the clock and independent of ground moisture. It would be desirable to conserve water by limiting the delivery of irrigation water when the ground moisture is adequate.
Prior soil moisture sensors have incorporated a pair of sensor fingers buried in the soil, together with a control system which applies a direct voltage applied thereacross. The application of direct current to the sensor fingers causes electrolysis and hydrolysis at the sensor. The hydrolysis and electrolysis effect breaks the bond between hydrogen and oxygen in the soil moisture. In turn, the hydrogen ion both causes conduction and appears to enhance the ionization of chloride in the chloride and other salts in the soil. The prior direct current type of sensor is not sensing water, but the ionized salts. Furthermore, the ionized salts are not necessarily proportional to the moisture in the soil in that region. The buildup due to electrolysis adjacent the sensor tends to increase the electrical transmissivity of that portion of the soil due to ionization. This increase in ionization in that region disturbs the reading so that the DC current through the probe is not proportional to the water moisture. The current stays the same while the soil becomes less moist over a period of time. Eventually, the sensor alloys and is conductive even though the soil is quite dry in the region of the sensor. Thus, the employment of direct current to sensor fingers is not reliable over a period of time, for example in the time period of six months to a year.